With a spread of digital cameras and mobile phones with a camera, the market for solid-state imaging devices has been recently expanded significantly. Furthermore, single lens reflex digital cameras in which various kinds of lenses ranging from wide-angle lenses to telescopic lenses can be exchanged in use are becoming popular.
Demands for slimmer digital cameras and the like have been still high. In other words, this means that the lens used in such a slimmer camera has a short focus, and that incident light to the solid-state imaging device has a wide angle (when measured with respect to the perpendicular axis of an incident surface of the solid-state imaging device).
A solid-state imaging device such as a charge coupled device (CCD) image sensor and a complementary metal oxide semiconductor (CMOS) image sensor includes a two-dimensional array of semiconductor integrated circuits (unit pixels) each including a light receiving unit, and thereby converts light signals from an object into electric signals.
Here, the solid-state imaging device has a sensitivity defined based on the magnitudes of current output from the light receiving elements with respect to the amounts of incident light. For this reason, reliably guiding incident light to the light receiving elements is a very important factor for increasing the overall sensitivity of the solid-state imaging device.
In order to solve a problem which occurs in the case of wide-angle incident light, a solid-state imaging device has been proposed which includes gradient index lenses having a fine structure smaller than or approximately the same as the wavelength of incident light and thus having an effective refractive index (see Patent Literature 1).
More specifically, the solid-state imaging device has, in the center part of the imaging region, such a gradient index lens that has an effective refractive index symmetrical to the center point of the unit pixel. The gradient index lens is formed with a combination of zones which respectively have the shapes of concentric arcs each having a line width smaller than or approximately the same as the wavelength of incident light.
More specifically, the solid-state imaging device has, in the marginal part of the imaging region, such a gradient index lens that has an effective refractive index asymmetrical to the center point of the unit pixel. The gradient index lens is formed with a combination of zones which respectively have the shapes of concentric arcs each having a line width smaller than or approximately the same as the wavelength of incident light, and the center point of the concentric arcs is shifted (offset) from the center point of the unit pixel.